Transmitting apparatus, receiving apparatus, and communication system for formatting data

ABSTRACT

A transmitting apparatus, a receiving apparatus, and a communication system are provided that allow a reduction in a frame loss due to interference caused by use of the same channel. A transmitting apparatus disposed in a base station includes a GPS receiver for receiving a GPS signal, a timing generator for controlling respective function blocks in accordance with the GPS signal and an inter-base-station control signal so as to precisely synchronize the timing of frame transmission among base stations, the front-end transmission processing unit including for converting transmission information into transmission time slots, a frame generator for generating a frame including a plurality of time slots and one frame guard, and a back-end transmission processing unit for transmitting the generated frame as a radio signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of Application No. 11/004,256, filedDec. 3, 2004, now U.S. Pat. No. 7,983,140, which is a continuation ofApplication No. 10/004,750, filed Dec. 3, 2001, now U.S. Pat. No.6,941,151, both of which are incorporated herein by reference, andclaims the benefit of Japanese Patent Application JP 2000-374606, filedDec. 8, 2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a transmitting apparatus, a receivingapparatus, and a communication system, for use in a mobile communicationsystem, and more specifically, to an improvement in a format of datathat is modulated and transmitted using, for example, an OFDM(Orthogonal Frequency Division Multiplexing) technique.

2. Description of the Related Art

In recent years, mobile communication using a portable telephone or thelike has become increasingly popular. Mobile communication is used totransmit not only information with a small data size such as voice databut also information with a large data size.

In a mobile communication system, as shown in FIG. 16, a plurality ofbase stations BS are distributed in a ground plane so that a mobilestation MS can communicate with a base station BS located near themobile station MS.

Herein, an area within which a base station can communicate with amobile station is referred to as a cell.

In such a mobile communication system, in order to avoid cross talk,each cell uses a frequency different from those used in adjacent cells.

However, the same frequency channel can be used in a more distant celloutside the adjacent cells without encountering a significant problem,because, for a mobile station MS being in a cell, the strength of asignal received from a base station BS of that cell is greater than thatof an interfering signal coming from a distant cell.

If the distance among cells in which the same frequency channel is usedis set to be very large, a large number of different frequency channelsare necessary, and thus the spectrum efficiency becomes low. That is,there is a trade-off between the interference due to usage of samefrequency channel and the spectrum efficiency.

Thus, it is important to design a communication system such that thesystem has high resistance against interference thereby achieving animprovement in the spectrum efficiency.

OFDM modulation is known as a technique having high resistance againstmultipath interference and having high spectrum efficiency.

In the OFDM modulation, after performing first modulation (such as QPSKor 16QAM), an inverse Fourier transform is performed on as manytransmission signal symbols as 2n at a time thereby creating as manyorthogonal subcarriers as 2n along a frequency axis as shown in FIG. 17.

In a mobile communication system using the OFDM modulation technique,each mobile station communicates with a base station closest to themobile station.

More specifically, in a communication system using the OFDM modulationtechnique, a plurality of time slots TSLT each including an effectivesymbol period TSBL and a guard period TGD are combined into a frame FRM,as shown in FIG. 18, and transmitted from a base station BS. In theexample shown in FIG. 18, each frame FRM includes three time slots.

Base stations BS are synchronized in terms of transmission so thatframes are transmitted with the same timing.

The purpose of a guard period TGD added to each effective symbol periodTSBL is to suppress intersymbol interference due to multipathtransmission or fading.

Each time slot including a guard period TGD is produced, as disclosed,for example, in Japanese Unexamined Patent Application Publication No.7-99486, by connecting the same signal as a predetermined length of heador tail end part of a signal in an effective symbol period to anopposite end of that effective symbol period or by connecting the samesignals as predetermined length of both head and tail end parts of asignal in an effective symbol period to opposite ends of the effectivesymbol period. More specifically, the same signal as a signal at a tailend part of an effective period is connected to the head end of theeffective symbol period, or the same signal as a signal at a head endpart of an effective period is connected to the tail end of theeffective symbol period, or otherwise, the same signals as signals athead and tail end parts of an effective period are respectivelyconnected to the tail and head ends of the effective symbol period.

In a receiving system of a mobile station that receives such an OFDMsignal, as shown in FIG. 19, the correlation is determined between thereceived OFDM signal and a signal obtained by delaying the OFDM signalby a time equal to one effective symbol period. The start positions ofrespective effective symbol periods are then determined from peakpositions of detected in the correlation. That is, it is possible todetermine the location of a guard period in each time slot.

The detection of the start position of an effective symbol period allowsan OFDM demodulator to perform an FFT (Fast Fourier Transform)operation.

An example of such an OFDM demodulator is disclosed in, for example,Japanese Unexamined Patent Application Publication No. 8-107431.

In the OFDM demodulator disclosed in the Japanese Unexamined PatentApplication Publication No. 8-107431 cited above, the correlationbetween a received OFDM signal and a signal obtained by delaying thereceived OFDM signal by an effective symbol period, and the resultantcorrelation signal is subjected to an interval integration. In the aboveprocess, the interval integration is performed, as shown in FIG. 20, forintervals created by dividing the correlation signal into segments thatis, intervals, each having a length equal to the time slot period.

That is, the cumulative sum of the correlation signal is determined byrepeatedly adding the correlation signal in the respective intervals. Inthe resultant signal, peaks appear at particular positions within thetime slot period as shown in FIG. 20(E). In parts where there is nocorrelation, the values are averaged as the interval integrationadvances.

As described above, the interval integration makes it possible toclearly distinguish a correlated period from an uncorrelated period, andthe detection of a peak makes it possible to achieve synchronization ina more reliable fashion.

In the communication system using an OFDM signal added with a guardperiod, as described above, although intersymbol interference due tomultipath transmission or fading can be suppressed, there is still apossibility that a mobile station encounters interference when receivingthe OFDM signal added with the guard period in some situations.

A mobile station receives a signal in such a manner as described below.

In addition to a desired wave DSW, a mobile station also receives aninterfering wave IFW via the same channel. In most cases, theinterfering wave IFW does not cause a problem, because the receptionsignal strength of the desired wave DSW is much greater than that of theinterfering wave IFW.

However, fading occurs as a mobile station moves, and thus the receptionsignal strength of the desired wave DSW and that of the interfering waveIFW frequently vary.

In general, there is no correlation between fading of a desired wave DSWand that of an interfering wave IFW. That is, the desired wave DSW andthe interfering wave IFW fluctuate independently of each other. Thismeans that the reception signal strength of the interfering wave IFW canbecome high when that of the desired wave DSW becomes low. In such acase, there is a possibility that interference makes it impossible toreceive the desired wave DSW.

In general, an interfering wave IFW arrives at a mobile station slightlylater than a desired wave DSW, because the interfering wave IFW istransmitted from a base station at a more distant location while thedesired wave DSW is transmitted from a base station at a closerlocation.

Referring to an example shown in FIG. 18, a possible reception of aninterfering wave IFW is discussed below for a case in which afluctuation in the reception signal strength due to fading causes asignal transmitted from a distant base station using the same channel tobe received as an interfering wave IFW. It is assumed herein that onlyone frame is received as the interfering wave IFW as shown in FIG.18(B).

In contrast, in the case of a desired wave DSW, frames are successivelyreceived as shown in FIG. 18(A).

Because the interfering wave IFW arrives slightly later than the desiredwave DSW as shown in FIG. 18(B), the interfering wave IFW interfereswith two frames, denoted by (i) and (ii) in FIG. 18, of the desired waveDSW.

In view of the above, an object of the present invention is to provide atransmitting apparatus, a receiving apparatus, and a communicationsystem, which allow suppression of a frame loss due to interferencecaused by use of the same channel even in a system in which the numberof repetition cells is set to be small, that is, the distance betweencells where the same channel is used is set to be small to achievehigh-efficiency use of radio channels.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, to achieve the aboveobject, there is provided a transmitting apparatus comprising afront-end transmission processing unit for converting transmissionsignal into a transmission time slot; a frame generator for generating aframe including a series of n (integer equal to or greater than 1) timeslots and a frame guard period added to the series of n time slots tosuppress a frame loss due to interference; and a back-end transmissionprocessing unit for transmitting the generated frame as a radio signal.

According to another aspect of the present invention, there is provideda transmitting apparatus disposed in at least one of a plurality of basestations each of which has a capability of communicating, using a signalaccording to a predetermined modulation scheme, with a communicationterminal being within an area assigned to the base station, thetransmitting apparatus comprising a front-end transmission processingunit for converting transmission signal into a transmission time slot; aframe generator for generating a frame including a series of n (integerequal to or greater than 1) time slots and a frame guard period added tothe series of n time slots to suppress a frame loss due to interference;and a back-end transmission processing unit for transmitting thegenerated frame as a radio signal.

The transmitting apparatus according to the present invention preferablyfurther comprises a timing generator for generating a timing signal onthe basis of a GPS signal and an inter-base-station control signal forachieving synchronization among base stations, thereby preciselysynchronizing the timing of frame transmission among the base stations.

In the transmitting apparatus according to the present invention, thefront-end transmission processing unit includes a modulator formodulating transmission information by means of a proper modulationscheme selected on the basis of electric field strength informationreceived from a communication terminal to which the transmissioninformation is transmitted.

In the transmitting apparatus according to the present invention, theframe guard period may be a non-signal period.

In the transmitting apparatus according to the present invention,preferably, the front-end transmission processing unit generates a timeslot by adding a predetermined guard period to an effective symbolperiod.

According to another aspect of the present invention, there is provideda receiving apparatus for receiving a radio signal, each frame of whichincludes a series of n (integer equal to or greater than 1) time slotsand a frame guard period added to the series of n time slots to suppressa frame loss due to interference, each time slot including an effectivesymbol period and a guard period added to the effective symbol period,the receiving apparatus comprising: a front-end reception processingunit for receiving the radio signal; a synchronization position detectorfor detecting a starting position of an effective symbol period in thereceived signal; a timing generator for controlling an operation timingof a functional block, on the basis of synchronization positioninformation supplied from the synchronization position detector; areception windowing unit for extracting only an effective symbol periodincluding no time guard period and no frame guard, under the control ofthe timing generator; and a back-end reception processing unit forreproducing desired information from a windowed signal supplied by thereception windowing unit.

According to still another aspect of the present invention, there isprovided a receiving apparatus disposed in a communication terminal forreceiving a radio signal transmitted from a base station each of whichhas a capability of communicating, using a signal according to apredetermined modulation scheme, with a communication terminal beingwithin an area assigned to the base station, each frame of the radiosignal including a series of n (integer equal to or greater than 1) timeslots and a frame guard period added to the series of n time slots tosuppress a frame loss due to interference, each time slot including aneffective symbol period and a guard period added to the effective symbolperiod, the receiving apparatus comprising: a front-end receptionprocessing unit for receiving the radio signal; a synchronizationposition detector for detecting a starting position of an effectivesymbol period in the received signal; a timing generator for controllingan operation timing of a functional block, on the basis ofsynchronization position information supplied from the synchronizationposition detector; a reception windowing unit for extracting only aneffective symbol period including no time guard period and no frameguard, under the control of the timing generator; and a back-endreception processing unit for reproducing desired information from awindowed signal supplied by the reception windowing unit.

According to still another aspect of the present invention, there isprovided a communication system comprising a transmitting apparatus anda receiving apparatus, wherein the transmitting apparatus comprises afront-end transmission processing unit for converting transmissionsignal into a transmission time slot; a frame generator for generating aframe including a series of n (integer equal to or greater than 1) timeslots and a frame guard period added to the series of n time slots tosuppress a frame loss due to interference, each time slot including aneffective symbol period and a guard period added to the effective symbolperiod; and a back-end transmission processing unit for transmitting thegenerated frame as a radio signal, and wherein the receiving apparatuscomprises a front-end reception processing unit for receiving a radiosignal transmitted from the transmitting apparatus; a synchronizationposition detector for detecting a starting position of an effectivesymbol period in the received signal; a timing generator for controllingan operation timing of a functional block, on the basis ofsynchronization position information supplied from the synchronizationposition detector; a reception windowing unit for extracting only aneffective symbol period including no time guard period and no frameguard, under the control of the timing generator; and a back-endreception processing unit for reproducing desired information from awindowed signal supplied by the reception windowing unit.

According to still another aspect of the present invention, there isprovided a communication system comprising a plurality of communicationterminals; and a plurality of base stations, each of which has acapability of communicating, using a signal according to a predeterminedmodulation scheme, with a communication terminal being within an areaassigned to the base station, wherein at least one of the plurality ofbase stations includes a transmitting apparatus comprising a front-endtransmission processing unit for converting transmission signal into atransmission time slot; a frame generator for generating a frameincluding a series of n (integer equal to or greater than 1) time slotsand a frame guard period added to the series of n time slots to suppressa frame loss due to interference, each time slot including an effectivesymbol period and a guard period added to the effective symbol period;and a back-end transmission processing unit for transmitting thegenerated frame as a radio signal, and wherein each communicationterminal includes a receiving apparatus comprising a front-end receptionprocessing unit for receiving a radio signal transmitted from thetransmitting apparatus; a synchronization position detector fordetecting a starting position of an effective symbol period in thereceived signal; a timing generator for controlling an operation timingof a functional block, on the basis of synchronization positioninformation supplied from the synchronization position detector; areception windowing unit for extracting only an effective symbol periodincluding no time guard period and no frame guard, under the control ofthe timing generator; and a back-end reception processing unit forreproducing desired information from a windowed signal supplied by thereception windowing unit.

In the communication system according to the present invention, thetransmitting apparatus preferably further comprises a timing generatorfor generating a timing signal on the basis of a GPS signal and aninter-base-station control signal for achieving synchronization amongbase stations, thereby precisely synchronizing the timing of frametransmission among the base stations.

In the communication system according to the present invention, thefront-end transmission processing unit of the transmitting apparatusincludes a modulator for modulating transmission information by means ofa proper modulation scheme selected on the basis of electric fieldstrength information received from a communication terminal to which thetransmission information is transmitted.

In the present invention, for example, the timing generator of atransmitting apparatus disposed in a base station generates a timingsignal from the GPS signal and the inter-base-station control signal sothat frames can be transmitted from any base station with the preciselysynchronized timing in accordance with the timing signal.

In the transmitting apparatus, the front-end transmission processingunit produces a transmission time slot from transmission information andsupplies the resultant transmission time slot to the frame generator.

The frame generator generates a frame including a plurality of timeslots and a non-signal period serving as a frame guard period, and theframe generator supplies the resultant frame to the back-endtransmission processing unit.

The back-end transmission processing unit transmits the supplied frameas a radio signal.

From each base station, as described above, a frame guard period isproduced in each transmission frame and the frame is transmitted withthe precisely synchronized timing.

If the front-end reception processing unit of the receiving apparatusdisposed in a mobile station receives the radio signal transmitted fromthe transmitting apparatus, the received radio signal is supplied to thesynchronization position detector.

The synchronization position detector detects the start position of aneffective symbol period from the received signal and outputssynchronization position information indicating the start position ofthe effective symbol period to the timing generator. The timinggenerator controls the operation timings of respective functional blockson the basis of the synchronization position information.

Under the control of the timing generator, the reception windowing unitextracts an effective symbol period including no time guard period andno frame guard period.

Thereafter, in the back-end reception processing unit, desiredinformation is reproduced from the windowed signal.

Thus, the received frame signal including the frame guard period isdemodulated and transmission information is reproduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a general construction of an OFDMcommunication system including a transmitting apparatus and a receivingapparatus according to the present invention;

FIG. 2 is a diagram illustrating a specific example of an OFDMcommunication system including a transmitting apparatus and a receivingapparatus according to the present invention;

FIGS. 3A to 3F are diagrams illustrating manners of forming cells in thecommunication system shown in FIG. 1;

FIG. 4 is a diagram illustrating an example of a manner of assigningradio channels according to an embodiment of the present invention;

FIG. 5 is a diagram illustrating an example of a format of an OFDMsignal including a frame guard according to the present invention;

FIG. 6 is a diagram illustrating a method of forming a time slot of anOFDM signal so as to include a guard, in accordance with the presentinvention;

FIG. 7 is a diagram illustrating a method of forming a time slot of anOFDM signal so as to include a guard, in accordance with the presentinvention;

FIG. 8 is a diagram illustrating a method of forming a time slot of anOFDM signal so as to include a guard, in accordance with the presentinvention;

FIG. 9 is a block diagram illustrating a transmitting apparatus disposedin a base station, according to an embodiment of the present invention;

FIG. 10 is a diagram illustrating symbol mapping according to 16QAM;

FIG. 11 is a diagram illustrating symbol mapping according to QPSK;

FIG. 12 is a diagram illustrating a process performed by a transmissionwindowing unit according to the present invention;

FIG. 13 is a schematic diagram illustrating a process performed by aframe generator according to the present invention;

FIG. 14 is a block diagram illustrating a receiving apparatus used in amobile station, according to an embodiment of the present invention;

FIG. 15 is a diagram illustrating an advantage obtained by using a frameguard;

FIG. 16 is a diagram illustrating a mobile communication system;

FIG. 17 is a diagram illustrating an OFDM modulation scheme;

FIG. 18 is a diagram illustrating an example of a conventional format ofan OFDM signal used in an OFDM transmission system;

FIG. 19 is a diagram illustrating a signal processing performed by areceiving system in a conventional mobile station; and

FIG. 20 is a diagram illustrating an interval integration performed by aconventional OFDM demodulator.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a general construction of an OFDM communicationsystem including a transmitting apparatus and a receiving apparatusaccording to the present invention. FIG. 2 illustrates a specificexample of an OFDM communication system including a transmittingapparatus and a receiving apparatus according to the present invention.

In this OFDM communication system 1, as shown in FIG. 1, a high-speeddownlink system is employed.

As shown in FIG. 1, the OFDM communication system 1 includes a mobilestation M1, a conventional base station B1, a high-speed downlink basestation B2, an existing cellular network (existing cellular cablenetwork) N1, a data communication network such as the Internet N2, and ahigh-speed downlink data communication network N3.

In FIGS. 1 and 2, the high-speed downlink system is denoted by “W-OFDM.”

In this OFDM communication system 1, as shown in FIG. 1, a controlsignal, such as an ARQ (Automatic Repeat Request) that is transmitted torequest for retransmission of a packet when a data error occurs, istransmitted via the conventional base station B1 and the network(cellular network) N1.

The high-speed downlink system has a very large transmission capacitycompared with a conventional portable telephone system so that a mobilestation M1 can download a digital content with a large data size, suchas image data or moving image data, in a short time via the high-speeddownlink system. Any information is transmitted in accordance with theIP.

The data communication network N3 for the high-speed downlink system isconnected to the data communication network N2 such as the Internet. Thedata communication network N3 for the high-speed downlink system is alsoconnected to network N1 so that various control signals generated by theportable telephone base station B1 are transmitted to the datacommunication network N3 via the network N1.

The OFDM communication system 1A shown in FIG. 2 mainly includes mobilestations (MS) M1 to M3, base satiations (BS) B1 to B4, an existingcellular cable network N1, a data communication network N2 such as theInternet, a data communication network N3 having a downlink database,and a control center (or mobile routing center) CTR for controlling theadditional downlink network.

The base station B1 functions as an existing cellular base station. Thebase station B2 has a capability of an additional downlink. The basestation B3 functions as an existing cellular base station. The basestation B4 has a capability of an additional downlink.

The cable network N1 is connected to the base stations B1 and B3, forexample, via cable communication lines L1 and L2.

The control center CTR is connected to the base stations B2 and B4 viacommunication lines L3 and L4.

The control center CTR is also connected to the network N1 via acommunication line L5, to the data communication network N2 via acommunication line L6, and to the data communication network N3 via acommunication line L7.

The OFDM communication system 1 or 1A is constructed so as to satisfythe following requirements.

That is, in recent years, mobile communication using a portabletelephone or the like has become increasingly popular, and mobilecommunication is used to transmit not only information with a small datasize such as voice data but also information with a large data size suchas a digital content.

In transmission of such digital data, it is required to transmitinformation with a much greater data size than information transmittedfrom an individual.

To handle such a large data size, an additional downstream line (i.e., adownlink for use of transmission in a direction from a base station to amobile station) is provided in such a manner that the downlink line isoverlaid on the existing cellular network.

This downlink is designed to be capable of transmitting a greater amountof information than the existing cellular network.

In this portable telephone communication system, a low bit rate signalsuch as a control signal from a mobile station of a user is transmittedusing the existing cellular network, and a high bit rate signal such asdigital data to be downloaded is transmitted at a high transmission ratevia the additional downlink.

In the OFDM communication system 1 or 1A, cells are constructed, forexample, as shown in FIG. 3.

In FIGS. 3(A) to 3(F), each solid line indicates an area (cell) withinwhich a conventional portable telephone base station can communicatewith a mobile station, and each dotted line indicates an area (cell)within which a wideband radio (W-OFDM) communication system, which isadditionally provided for exclusive use of downlink communication, cancommunicate with a mobile station.

More specifically, W-OFDM base stations may be disposed in the followingmanners. A first manner is, as shown in FIG. 3A, to dispose W-OFDM basestations in all cells where existing portable telephone base stationsare disposed so that the cell structure of W-OFDM base stations becomesimilar to the existing cell structure. A second manner is, as shown inFIG. 3B, to dispose W-OFDM base stations only in areas where there aremany users. A third manner is, as shown in FIG. 3C, to dispose, in areaswhere there are many users, W-OFDM base stations whose output power issmaller than existing base stations, so that the cell sizes covered byW-OFDM base stations become smaller (that is, micro cells are formed)than the existing cell sizes. A forth manner is, as shown in FIG. 3D, todispose W-OFDM base stations whose output power is greater than existingbase stations so that greater-sized cells are formed. A fifth manner is,as shown in FIG. 3E, a mixture of manners shown in FIGS. 3B and 3C(overlay system). A sixth manner is, as shown in FIG. 3F, to form microcells along main roads.

In the present embodiment, for example, the manner shown in FIG. 3A isemployed. That is, W-OFDM base stations are disposed in a similar manneras existing base stations so as to form cells similar to existing cells.

In the W-OFDM communication system 1A using the high-speed downlinksystem, base stations B1 to B4 receive a GPS (Global Positioning System)signal thereby achieving precise synchronization among those basestations B1 to B4.

An OFDM signal is transmitted in units of frames from a base station inthe W-OFDM communication system 1A, as will be described in detaillater, such that the timing of transmitting a frame is preciselysynchronized among all base stations.

In the W-OFDM communication system 1A, a frequency band is assigned thatis different from a frequency band assigned to the existing portabletelephone system.

The frequency band assigned to the W-OFDM communication system 1A isdivided into a plurality of radio channels, and divided radio channelsare assigned to respective base stations, for example, as shown in FIG.4 such that radio channels are used in an efficient manner whileminimizing interference caused by use of the same channel.

In the example shown in FIG. 4, the frequency band is divided into 12radio channels and assigned to base stations (cells). In FIG. 4,numerals from 1 to 12 enclosed in regular hexagons denote radio channelnumbers.

An example of a communication process performed in the OFDMcommunication system 1A shown in FIG. 2 is described below.

If a download request is issued from a mobile station, the downloadrequest is transmitted, via the existing cellular network N1 includingportable telephone base stations B1 and B3, to the control center CTR inthe network of the high-speed downlink system. Upon receiving thedownload request, the control center CTR transfer the download requestto the data communication network N2 such as the Internet. In responseto the download request, a digital data content is transmitted from thedata communication network N2 to the control center CTR, which in turntransfers the digital data content to the mobile station via the networkof the high-speed downlink system and further via the base stations B2and B4.

In the case where a data error occurs, an ARQ (Automatic Repeat Request)is issued from the mobile station and transmitted to the control centerCTR in the network of the high-speed downlink system via the existingcellular network including existing cellular base stations. In responseto the ARQ, the control center CTR re-transmits the requested digitaldata content to the mobile station via the network and base stations ofthe high-speed downlink system.

More specifically, for example, if the mobile station M1 issues a datadownload request to the control center CTR, a signal (001) is outputtedin a format adapted to the existing system from the mobile station M1and transmitted to the base station B1.

This request signal is then transmitted to the control center CTR viathe existing cellular network N1.

In response to receiving the data download request, the control centerCTR acquires the requested data (121) from the data communicationnetwork N2 via the communication line L6 and transmits the acquired data(121) as data (111), whose final destination is the mobile station M1,to the base station B2 via the communication line L3.

If the base station B2 receives this data (111), the base station B2transmits it as data (101) in a format adapted to the additionaldownlink to the mobile station M1.

Thus, the mobile station M1 finally receives the requested data (101).

In the case where, for example, a data download request to betransmitted to the control center CTR is issued by the mobile stationM3, a signal (003) is outputted in the format adapted to the existingsystem from the mobile station M3 and transmitted to the base stationB2.

This request signal is then transmitted to the control center CTR viathe existing cellular network N1.

In response to receiving the data download request, the control centerCTR acquires the requested data (123) from the data communicationnetwork N3 provided for exclusive use by the additional downlink via thecommunication line L7. To deliver the acquired data (123) to the mobilestation M3, the control center CTR transmits the acquired data (123) asdata (113) to the base station B4 provided for exclusive use by theadditional downlink via the communication line L4.

If the base station B3 receives the data (113), the base station B3transmits it as data (103) in the format adapted to the additionaldownlink to the mobile station M3.

Thus, the mobile station M3 finally receives the requested data (103).In the above-described OFDM communication system 1A, an OFDM signaltransmitted from a transmitting apparatus disposed in a base station toone of mobile stations M1 to M3 is generated such that each frame FRMincludes seven time slot periods TSLT and one frame guard period TFGD,as shown in FIG. 5.

In FIG. 5, TFRM, TSLT, and TFGD denote a frame period, a time slotperiod, and a frame guard period, respectively.

The frame guard period includes no signal, and, in the presentembodiment, is added to a frame FRM, at the end of a series of seventime slots.

The respective base stations B1 to B3 transmits signals in units offrames each including seven time slots SLT and one frame guard FGD suchthat the transmission timings become coincident with each other.

In the present embodiment, a frame guard period TFGD is placed at theend of a frame. Alternatively, a frame guard period TFGD may be placedat the beginning of a frame, or frame guard periods TFGD may be placedat both the beginning and the end of a frame.

Each time slot SLT included in a frame FRM is produced by adding a guardGD to an effective symbol period TSBL.

More specifically, each time slot SLT including an additional guard GDmay be produced, as shown in FIGS. 6 to 8, by connecting the same signalas a predetermined length of head or tail end part of a signal in aneffective symbol period to an opposite end of that effective symbolperiod or by connecting the same signals as predetermined length of bothhead and tail end parts of a signal in an effective symbol period toopposite ends of the effective symbol period. More specifically, in theexample shown in FIG. 6, the same signal as a signal at a tail end partof an effective period TSBL is connected to the head end of theeffective symbol period. In the example shown in FIG. 7, the same signalas a signal at a head end part of an effective period TSBL is connectedto the tail end of the effective symbol period, or otherwise. In theexample shown in FIG. 8, the same signals as signals at head and tailend parts of an effective period are respectively connected to the tailand head ends of the effective symbol period.

In the example shown in FIG. 5, the time slot is produced by the methodshown in FIG. 7.

As described above, the transmitting apparatus, for transmitting an OFDMsignal including a frame constructed by adding a guard period TFGD to aseries of time slots each including an effective symbol period TSBL anda guard period TGD added to the effective symbol period TSBL, isdisposed in a base station, and the receiving apparatus capable ofreceiving, with exactly synchronized timing, the OFDM signal includingthe additional frame guard period transmitted from the transmittingapparatus is provided in each of mobile stations M1 to M3.

Specific constructions and functions of a transmitting apparatusdisposed in a base station and a receiving apparatus disposed in amobile station are described below with reference to the drawings.

FIG. 9 is a block diagram illustrating a transmitting apparatus disposedin a base station, according to an embodiment of the present invention.

As shown in FIG. 9, the transmitting apparatus 100 according to thepresent embodiment includes a coder 101, an interleaver 102, a symbolmapper 103, a pilot signal inserter 104, a serial-parallel converter105, an IFFT unit 106, a parallel-serial converter 107, a time slotgenerator 108, a transmission windowing unit 109, a frame generator 110,a GPS receiver 111, a timing generator 112, a digital-analog (D/A)converter 113, a quadrature modulator 114, and a frequency converter115.

In this transmission apparatus 100, a front-end transmission processingunit is formed of the coder 101, the interleaver 102, the symbol mapper103, the pilot signal inserter 104, the serial-parallel converter 105,the IFFT unit 106, the parallel-serial converter 107, the time slotgenerator 108, and the transmission windowing unit 109, and a back-endtransmission processing unit is formed of the digital-analog (D/A)converter 113, the quadrature modulator 114, and the frequency converter115.

The coder 101 performs convolution coding with a constraint length of,for example, K=9 on digital data received via the network of thehigh-speed downlink system. The resultant coded data is outputted to theinterleaver 102. The mobile stations M1 to M3 monitor the strength ofthe electric field of a signal received from a base station of thehigh-speed downlink system. In accordance with the monitored electricfield strength, the coder 101 adjusts the coding rate within the range,for example, from R=2176/2488=0.8764 to R=44/1370=0.397.

The interleaver 102 interleaves the coded digital data supplied from thecoder 101 and outputs the resultant interleaved data to the symbolmapper 103.

The symbol mapper 103 determines the symbol mapping scheme (scheme ofthe first modulation) in accordance with the strength of the electricfield, monitored by the mobile station, of the signal transmitted fromthe base station of the high-speed downlink system, and the symbolmapper 103 performs symbol mapping by the determined symbol mappingscheme. The resultant symbol-mapped data including an I-channel signaland a Q-channel signal is outputted to the pilot signal inserter 104.

For example, when the electric field strength of the signal transmittedfrom the base station of the high-speed downlink system has a stablehigh value, the symbol mapper 103 employs, as the modulation scheme, the16QAM symbol mapping scheme. In the 16QAM symbol mapping scheme, symbolmapping is performed as shown in FIG. 10. On the other hand, when theelectric field strength is weak or unstable, the QPSK (Quadrature PhaseShift Keying) or DQPSK (Differential QPSK) scheme is employed as themodulation scheme. In this case, symbol mapping is performed as shown inFIG. 11.

The pilot signal inserter 104 inserts a pilot signal of “1” into theI-channel signal supplied from the symbol mapper 103 and a pilot signalof “0” into the Q-channel signal and outputs the resultant signals tothe serial-parallel converter 105.

The pilot signals inserted by the pilot signal inserter 104 are used bya receiving apparatus of a mobile station to estimate a transmissionpath and to make a phase compensation. The pilot signals are also usedto calculate a threshold value used as a reference value of an amplitudein the first modulation process based on a modulation scheme such as16QAM in which information is represented by the amplitude.

The serial-parallel converter 105 converts the symbol data including theinserted pilot signals from serial form into parallel form and outputsthe resultant data to the IFFT unit 106.

More specifically, the serial-parallel converter 105 divides the inputsymbol data into segments every 98 symbols, and adds one symbol to headand tail ends of each segment so that each segment includes 100 symbols.Furthermore, the serial-parallel converter 105 puts 1948 symbols of “0”before and after each segment including 100 symbols so that each segmentincludes a total of 2048 symbols and so that the resultant symbol datahas a frequency spectrum in a radio channel band assigned to a basestation. The resultant parallel symbol data is outputted to the IFFTunit 106.

The IFFT unit 106 performs an IFFT operation for 2048 points. Morespecifically, the IFFT unit 106 performs an inverse fast Fouriertransform on the parallel 2048 symbol data outputted from theserial-parallel converter 105 thereby making a conversion between timeand frequency domains. The resultant data is outputted to theparallel-serial converter 107.

In the OFDM signal used in the present embodiment, the subcarrierinterval is, for example, 4 KHz and the effective symbol period is equalto the reciprocal of the subcarrier repetition frequency, that is, equalto 250 (s. The OFDM signal can include a variable number of subcarriersin units of 100 subcarriers (with a frequency bandwidth of 400 kHz) upto 1600 subcarriers (with a frequency bandwidth of 400 kHz (16=6.4 MHz)The IFFT unit performs the IFFT operation for 2048 points.

Herein, we assume that a base station is assigned a radio channel with abandwidth of 400 kHz.

In this case, as described above, the symbol data inputted to theserial-parallel converter 105 is divided into segments every 98 symbols,and one symbol is added to the head end and also to the tail end of eachsegment so that each segment includes 100 symbols. Furthermore, 1948symbols of “0” are put before and after each segment including 100symbols so that each segment includes a total of 2048 symbols and sothat the resultant symbol data has a frequency spectrum in a radiochannel band assigned to the base station. The resultant parallel symboldata is inputted to the IFFT unit 106, which performs the IFFT operationfor 2048 points, that is, performs the inverse fast Fourier transform onthe inputted data thereby making a conversion between time and frequencydomains.

The parallel-serial converter 107 converts the parallel data suppliedfrom the IFFT unit 106 into serial data thereby obtaining time seriesdata including 2048 points. The resultant serial data is outputted tothe time slot generator 108.

In the present embodiment, the system clock is set to have a frequencyof 8.192 MHz. Therefore, the length (effective symbol period) of timeseries data including 2048 points becomes (1/8.192 (106) (2048=250 (10-6sec.

The time slot generator 108 generates a time slot, for example, as shownin FIG. 8, by connecting data including 120 points (14.648 (s), whichare the same as 120 points at the head end of the time series dataincluding 2048 points in the effective symbol period, to the tail endand further connecting data including 120 points that are the same as120 points at the tail end to the head end. The generated time slot isoutputted to the transmission windowing unit 109.

Alternatively, as shown in FIG. 7, the time slot generator 108 generatesa time slot by connecting data including 240 points (29.297 (s) that arethe same as 240 points at the head end of the effective symbol periodincluding 2048 points to the tail end of the effective symbol period andoutputs the generated time slot to the transmission windowing unit 109.

The transmission windowing unit 109 performs windowing on the time slotgenerated by the time slot generator 108, for example, as shown in FIG.12, such that a ramp period dTx is added to the head end and also to thetail end of the time slot period TSLT. The resultant time slot isoutputted to the frame generator 110.

In the present embodiment, the ramp periods dTx put at the head ant tailends each has a length of 2.44 (s, and thus the total length is equal to4.88 (s. The purpose of these ramp periods dTx is to prevent undesirableleakage of spectrum to the outside of the assigned frequency band.

The frame generator 110 generates one frame FRM, for example, as shownin FIG. 13, by combining seven time slots and putting, thereafter, a0-power non-signal period (frame guard period) with a lengthcorresponding to 368 points (44.92 (s). The generated frame FRM isoutputted to the digital-analog (D/A) converter 113.

As shown in FIG. 13, the length of one time slot period TSLT is equal to2288 points (279.3 (s), and thus the length of one frame period TFRMincluding seven time slots and one frame guard period TFGD becomes equalto 16384 points (2 ms).

The GPS receiver 111 receives a GPS signal via a receiving antenna 111 aand outputs the received GPS signal to the timing generator 112.

On the basis of the GPS signal supplied from the GPS receiver 111 andthe inter-base-station control signal CTL, the timing generator 112generates a timing signal for controlling the transmission timing of theframe generator 110. The generated timing signal S112 is outputted tothe frame generator 110.

In the present embodiment, as described earlier, all base stationstransmit frames so that the transmission timings become coincident witheach other. To this end, each base station transmits frames insynchronization with the inter-base-station control signal CTL.

This synchronization signal is transmitted via the cable communicationnetwork. However, in the case where only this synchronization signal isused, precise synchronization among base stations cannot be achieved,because the synchronization signal encounters a propagation delay whenit is transmitted over the cable network. To achieve precisesynchronization, each base station also receives the GPS signal andcontrols the frame transmission timing in accordance with the GPS signaland the inter-base-station control signal CTL.

The D/A converter 113 converts the digital frame data generated by theframe generator 110 into an analog data and outputs the resultant analogdata to the quadrature modulator 114.

The quadrature modulator 114 performs quadrature modulation, inaccordance with a predetermined scheme, on the frame data that is to betransmitted and that has been converted by the D/A converter 114 intoanalog form. The resultant data is outputted to the frequency converter115.

The frequency converter 115 converts the quadrature-modulated datasupplied from the quadrature modulator 114 so as to have a frequency ina predetermined frequency band. The resultant signal is transmitted as aRF (Radio Frequency) signal.

FIG. 14 is a block diagram illustrating a receiving apparatus used in amobile station, according to an embodiment of the present invention.

As shown in FIG. 14, the receiving apparatus 200 according to thepresent embodiment includes a frequency converter 201, a quadraturedemodulator 202, an analog-digital (A/D) converter 203, asynchronization position detector 204, a timing generator 205, areception windowing unit 206, a serial-parallel converter 207, a FFTunit 208, a parallel-serial converter 209, a transmission path estimator210, a phase compensator 211, a demodulator 212, a deinterleaver 213,and a decoder 214.

In this receiving apparatus 200, a front-end reception processing unitis formed of the frequency converter 201, the quadrature demodulator202, and the A/D converter 203, and a back-end reception processing unitis formed of the serial-parallel converter 207, the FFT unit 208, theparallel-serial converter 209, the transmission path estimator 210, thephase compensator 211, the demodulator 212, the deinterleaver 213, andthe decoder 214.

The frequency converter 210 extracts only components within a necessaryfrequency band from an OFDM signal received via an antenna (not shown),that is, the frequency converter 210 removes noise components outsidethe necessary frequency bands, and then converts the resultant RF signalinto an IF (Intermediate Frequency) signal. The resultant IF signal S201is outputted to the quadrature demodulator 202.

The quadrature demodulator 202 separates an in-phase signal I and aquadrature signal Q from the IF signal supplied from the frequencyconverter 201 and outputs them to the A/D converter 203.

The A/D converter 203 converts the in-phase signal I and the quadraturesignal Q supplied from the quadrature demodulator 202 from analog forminto digital form. The resultant digital signals are outputted to thesynchronization position detector 204 and the reception windowing unit206.

In the above process, the sampling rate employed by the A/D converter203 is set to be equal to 8.192 MHz so that the sampling rate becomesequal to that employed by the transmitting apparatus 100 in the basestation.

The synchronization position detector 204 detects the FFT operationtiming of the FFT unit 208, from the I- and Q-signals converted intodigital form. That is, the synchronization position detector 204 detectsthe start position of an effective symbol period TSBL, in other words,the position of the first point in the digital signal in the effectivesymbol period TSBL. The synchronization position detector 204 outputsthe resultant synchronization information to the timing generator 205.

On the basis of the synchronization information supplied from thesynchronization position detector 204, the timing generator 205 controlsthe start of the reception windowing operation performed by thereception windowing unit 206, the serial-parallel conversion positionperformed by the serial-parallel converter 207, the timing of the FFToperation performed by the FFT unit 208, and the timing of theparallel-serial conversion performed by the parallel-serial converter209.

On the basis of the digital signal supplied from the A/D converter 203and the windowing start position information supplied from the timinggenerator 205, the reception windowing unit 206 extracts 2048 points ofdigital data (250 (s) starting from the synchronization point andoutputs the extracted data to the serial-parallel converter 207.

Compared with the transmission window (279.3 (s) provided by thetransmission windowing unit 109 of the base station transmittingapparatus 100, the reception window (250 (s) provided by the receptionwindowing unit 206 is small in length.

The serial-parallel converter 207 converts the 2048 points of digitaldata supplied from the reception windowing unit 206 from serial forminto parallel form and outputs the resultant parallel data to the FFTunit 208.

In accordance with the FFT timing information supplied from the timinggenerator 205, the FFT unit 208 performs a fast Fourier transform on the2048 points of digital data thereby making a conversion between thefrequency domain and the time domain. The resultant data is thensupplied to the parallel-serial converter 209.

Thus, via the fast Fourier transform, the signal in the form of a timeseries signal including 2048 points and having a spectrum of 100 (n (1(n (16) subcarriers located in intervals of 4 KHz is converted into adigital signal including 100 (n (1 (n (16) points.

In practice, a digital signal including 2048 points is obtained as aresult of fast Fourier transform for 2048 points. However, the availablesystem frequency bandwidth is limited to 6.4 MHz. Therefore, of 2048subcarriers, up to 1600 subcarriers are used by the transmittingapparatus disposed in a base station, and the remaining 448 subcarriersare set to “0” in power. Thus, the digital signal, which is actuallyoutputted, includes up to 1600 subcarriers, and the remainingsubcarriers have a value of “0”.

The parallel-serial converter 209 converts the parallel signal suppliedfrom the FFT unit 208 into a serial signal and extracts only necessarypoints from the 2048 points. The resultant 2048 points of data areoutputted to the transmission path estimator 210.

For example, in the case where a frequency bandwidth of 400 kHz isassigned for communication between this mobile station and the basestation, the parallel-serial converter 209 at a receiving end extractsonly 100 points corresponding to the bandwidth of 400 kHz.

The signal outputted from the parallel-serial converter 209 is outputtedto the transmission path estimator 210. Upon receiving the signal fromthe parallel-serial converter 209, the transmission path estimator 210extracts only the pilot signal from the received signal and calculatesthe phase shift from the I-channel and Q-channel components of the pilotsignal. The signal indicating the phase shift is outputted to the phasecompensator 211.

More specifically, because the base station transmitting apparatus 100transmits the pilot signal such that the I-channel component thereof hasa level of “1”, and the Q-channel component has a level of “0”, thepilot signal represented in a complex plane has a magnitude of “1” and aphase angle of “0” with respect to the I axis. Therefore, the I and Qvalues in the complex plane obtained in receiving apparatus 200 of themobile station directly indicate the phase shift.

The information about the magnitude of the vector in the complex planeis used to determine the threshold value used in the demodulation of asignal modulated by means of a multilevel modulation such as 16QAM.

The phase compensator 211 corrects the phase of the received signal onthe basis of the information about the phase shift detected by thetransmission path estimator 210. The resultant phase-compensated signalis outputted to the demodulator 212.

The demodulator 212 demodulates the signal in accordance with thedemodulation scheme corresponding to the modulation scheme employed bythe transmitting apparatus 100 of the base station. The demodulatedsignal is outputted to the deinterleaver 213.

In the case where a modulation scheme such as 16QAM in which informationis represented by the amplitude (magnitude of the vector represented inthe complex plane) is employed, the transmission path estimator 210provides information about the reference reception power level(magnitude of the vector of the received pilot signal), and demodulationis performed using the reference reception power level provided by thetransmission path estimator 210.

The deinterleaver 213 deinterleaves the demodulated signal supplied fromthe demodulator 212 and outputs the resultant signal to the decoder 214.

If the decoder 214 receives the signal that has been demodulated anddeinterleaved, the decoder 214 performs, for example, Viterbi decodingon the received signal. Thus, a decoded signal is finally obtained.

The operations of the transmitting apparatus and receiving apparatusused in the OFDM communication system constructed in the above-describedmanner are described below.

When the transmitting apparatus 100 of a base station receives digitaldata via, for example, the high-speed downlink network, the coder 101performs convolution coding with a constraint length of K=9 on thereceived digital data.

The coded digital data outputted from the coder 101 is interleaved bythe interleaver 102 and inputted to the symbol mapper 103.

The symbol mapper 103 determines the symbol mapping scheme (scheme ofthe first modulation) in accordance with the strength of the electricfield, monitored by the mobile station, of the signal transmitted fromthe base station of the high-speed downlink system, and the symbolmapper 103 performs symbol mapping by the determined symbol mappingscheme. The resultant symbol-mapped data including an I-channel signaland a Q-channel signal is outputted to the pilot signal inserter 104.

The pilot signal inserter 104 inserts a pilot signal of “1” into theI-channel signal supplied from the symbol mapper 103 and a pilot signalof “0” into the Q-channel signal and outputs the resultant signals tothe serial-parallel converter 105.

The serial-parallel converter 105 divides the input symbol data intosegments, for example, every 98 symbols, and adds one symbol to head andtail ends of each segment so that each segment includes 100 symbols.Furthermore, the serial-parallel converter 105 puts 1948 symbols of “0”before and after each segment including 100 symbols so that each segmentincludes a total of 2048 symbols and so that the resultant symbol datahas a frequency spectrum in a radio channel band assigned to a basestation. The resultant parallel symbol data is outputted to the IFFTunit 106.

The IFFT unit 106 performs an inverse fast Fourier transform on theparallel 2048 symbol data outputted from the serial-parallel converter105 thereby making a conversion between time and frequency domains. Theresultant data is outputted to the parallel-serial converter 107.

The parallel-serial converter 107 converts the parallel data outputtedfrom the IFFT unit 106 into serial data thereby generating time seriesdata including 2048 points. The resultant serial data is outputted tothe time slot generator 108.

The time slot generator 108 generates a time slot, for example, byconnecting data including 120 points (14.648 (s), which are the same as120 points at the head end of the time series data including 2048 pointsin the effective symbol period, to the tail end and further connectingdata including 120 points that are the same as 120 points at the tailend to the head end. The generated time slot is outputted to thetransmission windowing unit 109.

The transmission windowing unit 109 performs windowing on the time slotgenerated by the time slot generator 108, for example, such that a rampperiod dTx is added to the head end and also to the tail end of the timeslot period TSLT to prevent undesirable leakage of spectrum to theoutside of the assigned frequency band. The resultant time slot isoutputted to the frame generator 110.

The frame generator 110 generates one frame FRM, for example, bycombining seven time slots and putting, thereafter, a 0-power non-signalperiod (frame guard period) with a length corresponding to 368 points(44.92 (s). The generated frame FRM is outputted to the digital-analog(D/A) converter 113.

Each base station transmits frames in synchronization with theinter-base-station control signal CTL.

This synchronization signal is transmitted via the cable communicationnetwork. However, in the case where only this synchronization signal isused, precise synchronization among base stations cannot be achieved,because the synchronization signal encounters a propagation delay whenit is transmitted over the cable network. Thus, in each base station, toachieve precise synchronization, the GPS receiver 111 receives the GPSsignal, and, on the basis of the GPS signal and the inter-base-stationcontrol signal CTL, the timing generator 112 generates a timing signalfor controlling the timing of frame transmission performed by the framegenerator 110 and outputs the resultant timing signal S112 to the framegenerator 110.

The frame generator 110 generates a frame at a time specified by thetiming signal S112 and outputs the generated frame to the D/A converter113.

The D/A converter 113 the digital frame data generated by the framegenerator 110 into analog data. The resultant analog data is thentransmitted after being quadrature-modulated by the quadrature modulator114 in accordance with a predetermined modulation scheme and convertedby the frequency converter 115 into a frequency in a predeterminedfrequency band.

The OFDM signal transmitted by the transmitting apparatus 100 of thebase station is received by the receiving apparatus 200 of a mobilestation.

The signal received by the receiving apparatus 200 is passed through abandpass filter (not shown) to extract only components in a necessaryfrequency band and is converted into an IF signal by the frequencyconverter 201. Thereafter, the IF signal is separated by the quadraturedemodulator into an I-signal and a Q-signal. The resultant I-signal andQ-signal are converted into digital form by the A/D converter 203.

After analog-digital conversion, both the I-signal and the Q-signal aresupplied to the synchronization position detector 204 and the receptionwindowing unit 206.

The synchronization position detector 204 detects the timing of the FFToperation to be performed by the FFT unit 208. That is, thesynchronization position detector 204 detects the start position of aneffective symbol period TSBL, in other words, the position of the firstpoint in the digital signal in the effective symbol period. Thesynchronization position detector 204 outputs the resultantsynchronization information to the timing generator 205.

On the basis of the synchronization information supplied from thesynchronization position detector 204, the timing generator 205 controlsthe start of the reception windowing operation performed by thereception windowing unit 206, the serial-parallel conversion positionperformed by the serial-parallel converter 207, the timing of the FFToperation performed by the FFT unit 208, and the timing of theparallel-serial conversion performed by the parallel-serial converter209.

On the basis of the digital signal supplied from the A/D converter 203and the windowing start position information supplied from the timinggenerator 205, the reception windowing unit 206 extracts 2048 points ofdigital data (250 (s) starting from the synchronization point andoutputs the extracted data to the serial-parallel converter 207.

The serial-parallel converter 207 converts the 2048 points of digitaldata supplied from the reception windowing unit 206 from serial forminto parallel form and outputs the resultant parallel data to the FFTunit 208.

In accordance with the FFT timing information supplied from the timinggenerator 205, the FFT unit 208 performs a fast Fourier transform on the2048 points of digital data thereby making a conversion between thefrequency domain and the time domain. The resultant data is thensupplied to the parallel-serial converter 209.

The parallel-serial converter 209 converts the parallel signal suppliedfrom the FFT unit 208 into a serial signal. Via this, process,particular points are extracted from 2048 points.

The transmission path estimator 209 extracts only the pilot signal fromthe received signal and calculates the phase shift from the I-channeland Q-channel components of the pilot signal. The signal indicating thephase shift is outputted to the phase compensator 211.

The phase compensator 211 corrects the phase of the received signal onthe basis of the phase shift information and supplies the correctedsignal to the demodulator 212.

The demodulator 212 demodulates the signal in accordance with thedemodulation scheme corresponding to the modulation scheme employed bythe transmitting apparatus of the base station. In the case where amodulation scheme such as 16QAM in which information is represented bythe amplitude (magnitude of a vector represented in the complex plane)is employed, the transmission path estimator 210 provides informationabout the reference reception power level (magnitude of the vector ofthe received pilot signal), and demodulation is performed using thereference reception power level provided by the transmission pathestimator 210.

The deinterleaver 213 deinterleaves the demodulated signal supplied fromthe demodulator 212 and outputs the resultant signal to the decoder 214.The decoder 214 performs Viterbi decoding on the received signal.

The contribution of an interfering wave in the present communicationsystem is discussed below with reference to FIG. 15. Herein, we assumethat one frame of interfering wave has arrived due to a fluctuation inthe electric field strength of the reception signal caused by fading ormultipath transmission.

In this situation, a plurality of frames are being transmittedsuccessively as a desired wave DSW. Because all base stations transmitframes with the precisely synchronized timing, the interfering wave IFWtransmitted from a distant base station arrives slightly later than thedesired wave DSW using the same channel transmitted from a base stationat a closer location.

In the convention technique in which no frame guard is used, theinterfering wave IFW interferes with two frames of the desired wave DSW.In contrast, in the communication system according to the presentembodiment of the invention, a frame guard included in an OFDM signalprevents the interfering wave IFW from interfering with the secondframe, as shown in FIGS. 15(A) and 15(B).

In the present embodiment, as described above, to transmit the frameincluding the additional frame guard with the precisely synchronizedtiming from each base station, the base-station transmitting apparatus100 is constructed so as to include the inter-base-station controlsignal interface for achieving synchronization among base stations, thereceiving antenna 111 a for receiving the GPS signal, the GPS receiver111 for receiving the GPS signal, the timing generator 112 forcontrolling the respective function blocks in accordance with the GPSsignal and the inter-base-station control signal CTL so as to preciselysynchronize the timing of frame transmission among the base stations,the front-end transmission processing unit including the blocks 101 to109 for converting transmission information into transmission timeslots, the frame generator 110 for generating a frame including aplurality of time slots and one frame guard, and the back-endtransmission processing unit including the blocks 113 to 115 fortransmitting the generated frame as a radio signal, and, to demodulate areceived signal including a frame guard period thereby reproducingtransmission information, the receiving apparatus 200 is constructed soas to include the front-end reception processing unit including blocks201 to 203 for receiving a radio signal and converting the receivedradio signal into a digital signal; the synchronization positiondetector 204 for detecting the start position of an effective symbolperiod from the received signal; the timing generator 205 forcontrolling the operation timings of respective functional blocks on thebasis of the synchronization position information; the receptionwindowing unit 206 for extracting only the effective symbol periodincluding no time guard period and no frame guard period under thecontrol of the timing generator; and the back-end reception processingunit including blocks 207 to 214 for reproducing desired informationfrom the windowed signal, thereby ensuring that a frame loss due tointerference caused by use of the same channel can be suppressed even ina system in which the number of repetition cells is set to be small,that is, the distance between cells where the same channel is used isset to be small to achieve high-efficiency use of radio channels.

Thus, it is possible to reduce the number of repetition cells withoutcausing an increase in a transmission error, thereby achieving efficientuse of frequency resources.

Furthermore, it is possible to achieve an improvement in synchronizationin the OFDM radio communication system using a frame guard.

Using the synchronization apparatus, it is possible to determine a pointat which a frame guard should be inserted. This makes it unnecessary totransmit frame synchronization control information (indicating the startposition of a frame), and thus it becomes possible to transmit anincreased amount of information.

As described above, the present invention provides great advantages.That is, a frame loss due to interference caused by use of the samechannel can be suppressed even in a system in which the number ofrepetition cells is set to be small, that is, the distance between cellswhere the same channel is used is set to be small to achievehigh-efficiency use of radio channels.

Thus, it is possible to reduce the number of repetition cells withoutcausing an increase in a transmission error, thereby achieving efficientuse of frequency resources.

Furthermore, the present invention makes it possible to achievesynchronization in a radio communication system using a frame guard.

Furthermore, because it is possible to determine a point at which aframe guard should be inserted, it is unnecessary to transmit framesynchronization control information (indicating the start position of aframe), and thus it becomes possible to transmit an increased amount ofinformation.

What is claimed is:
 1. A transmission method performed in an orthogonal frequency division multiplexing wireless communication system, the method comprising: converting a transmission signal into a transmission time slot; generating a frame that includes a series of n (greater than 1) time slots and a frame guard period added to the series of n time slots, the frame guard period being a non-signal period, and each time slot including an effective symbol period and guard period added to the effective symbol period, where the length of the series of n time slots is less than the length of the frame, wherein the frame guard period is separate from the guard period; and transmitting the generated frame as a radio signal using a transmission apparatus, wherein the radio signal is an OFDM signal having the frame guard period.
 2. A transmission method according to claim 1, further comprising: modulating transmission information to produce the OFDM signal using a modulation scheme selected on the basis of electric field strength information received from a communication terminal to which the transmission information is transmitted.
 3. A transmission method according to claim 1, further comprising: generating a time slot by adding a predetermined guard period to an effective symbol period.
 4. The transmission method of claim 1, wherein the frame guard period is at the beginning and/or the end of the frame.
 5. A transmission system comprising: a front-end transmission processing unit for converting a transmission signal into a transmission time slot; a frame generator for generating a frame that includes a series of n (greater than 1) time slots and a frame guard period added to the series of n time slots, the frame guard period being a non-signal period, and each time slot including an effective symbol period and guard period added to the effective symbol period, where the length of the series of n time slots is less than the length of the frame, wherein the frame guard period is separate from the guard period; and a back-end transmission processing unit for transmitting the generated frame as a radio signal, wherein the radio signal is an OFDM signal having the frame guard period.
 6. A transmission system according to claim 5, wherein the front-end transmission processing unit includes a modulator for modulating transmission information to produce the OFDM signal using a modulation scheme selected on the basis of electric field strength information received from a communication terminal to which the transmission information is transmitted.
 7. A transmission system according to claim 5, wherein the front-end transmission processing unit generates a time slot by adding a predetermined guard period to an effective symbol period.
 8. The transmission system of claim 5, wherein the frame guard period is at the beginning and/or the end of the frame.
 9. A computer program, embodied in a non-transitory computer-readable medium, when executed on one or more processors, causing the one or more processors to perform steps comprising: converting a transmission signal into a transmission time slot; generating a frame that includes a series of n (greater than 1) time slots and a frame guard period added to the series of n time slots, the frame guard period being a non-signal period, and each time slot including an effective symbol period and guard period added to the effective symbol period, where the length of the series of n time slots is less than the length of the frame, wherein the frame guard period is separate from the guard period; and transmitting the generated frame as a radio signal, wherein the radio signal is an OFDM signal having the frame guard period.
 10. A computer program according to claim 9, further causing the one or more processors to perform the step of: modulating transmission information to produce the OFDM signal using a modulation scheme selected on the basis of electric field strength information received from a communication terminal to which the transmission information is transmitted.
 11. A computer program according to claim 9, further causing the one or more processors to perform the step of: generating a time slot by adding a predetermined guard period to an effective symbol period.
 12. The computer program of claim 9, wherein the frame guard period is at the beginning and/or the end of the frame. 